The construction of many prior art skis include an outer skin or casing filled with a core material different from the outer skin. Typically, the outer skin is formed of a fiber-reinforced plastic such as fiberglass or some other laminate material. Typical core materials that have been used include wood or expanded plastic material. The rigidity of such skis generally depends upon the core material selected and/or the thickness of the skin. Rigidity of prior art skis either required a hardwood core such as an oak core or an increase in the thickness of the skin. Using a hardwood core greatly increases the weight of the ski, thus reducing its buoyancy. A reduction in buoyancy increases the drag of the ski since the ski sits deeper in the water and the ski provides greater resistance through the water instead of the ski gliding along the water surface. Increasing the thickness of the skin also increases both the weight and buoyancy of the ski.
Additionally, a substantial increase in skin thickness is required to have much effect on the longitudinal rigidity of the ski. This is because the skin is a thin sheet. Thin sheets are not very resilient when subject to forces normal to a face of the sheet. For example, a thin sheet of aluminum, when subject to a particular force, will flex a greater amount than will an aluminum beam having the same mass as the sheet.
Some skis have incorporated means for increasing torsional rigidity. By torsional rigidity, it is meant the ability of the ski to oppose twisting when subject to transverse forces. U.S. Pat. No. 4,094,528 to Cluzel discloses a ski requiring a trapezoidal core of foam material which is wrapped in a fiber-reinforced plastic material. The purpose of the trapezoidal core is to provide greater resistance to lateral forces. A disadvantage of the Cluzel ski is that the trapezoidal member increases both the torsional rigidity and the longitudinal rigidity simultaneously. To increase the torsional rigidity, the trapezoidal foam material is wrapped in a greater amount of plastic material. However, this also increases the longitudinal rigidity. Since the torsional rigidity cannot be selected independent of the longitudinal rigidity, independent characteristics of the ski cannot be selected for each individual skier.
Another disadvantage of the skis described above is that each ski's resonant frequency cannot be selected independent of its rigidity. By resonant frequency, it is meant the frequency which causes the greatest vibration on the ski. A ski traveling on water passes over ripples in the water. If the ski strikes the ripples at a rate equal to the resonant frequency, the ski will vibrate. Such vibration is undesirable because it reduces the user's control of the ski and also fatigues the user.